Hearing instrument acoustic gain is primarily limited by feedback of the output signal back to the input of the device. Mild amounts of feedback change the transfer function of the system, thereby coloring the sound output. Larger amounts of feedback will cause instability and oscillation (squealing noises). There are multiple paths for feedback, one of which is the mechanical vibration of the hearing instrument receiver. The case of the receiver vibrates in reaction to the motion of the internal parts. This vibration, in turn, couples to the diaphragm in the hearing instrument microphone, either directly, or indirectly through the hearing instrument case moving the air that is near the microphone.
The vibration of receivers can be largely cancelled out by connecting a pair of receivers together such that their primary direction of vibration is in opposition. The motion of the two devices then cancels, greatly reducing the net vibration. Unfortunately, the receivers have vibration components in both the vertical and horizontal directions. When two receivers are combined together, the vertical components cancel, but the horizontal portion adds to the vibration.
Thus, there is a need for receivers that have vibration strictly in the vertical direction, with no vibration in the horizontal direction. There is also a need to make the hearing instrument as small as possible, to improve the fit of the device into the ear canal, or to reduce the visibility of the instrument. The need for a smaller sized hearing instrument creates a need for a smaller sized receiver. The receiver size can be reduced by folding the armature, such as that design seen in known receivers. Armatures 2, 4, 6 which are used in balanced armature receivers typically use a U or E shaped armature (see FIGS. 1-3), where one portion of the armature is free to move, and the other portion is fixed to a magnetic yoke to complete the magnetic circuit. To carry the maximum amount of magnetic signal, the cross-sectional area of the fixed portion must be at least as large as the moving portion. If there is less area, then the maximum level of sound that the receiver can produce may be reduced.
A flat E-shaped armature 4, as shown in FIG. 2, has no vibration along its length at the primary frequency of motion. The only lengthwise vibration is a component at double the frequency of the primary motion. This double frequency is benign, as it does not contribute to feedback in the hearing instrument. However, the armature takes up significant width. The outer tines 7 and the connecting portion 9 of the armature 6 are typically folded perpendicular to the moving portion to save space, as shown in FIG. 3. This introduces a tradeoff of width and vibration. In other embodiments, if the connection portion is folded out of the plane of the moving portion of the armature, the motion of the connection portion may introduce an unwanted horizontal component to the receiver vibration. Folding just the sides, as shown in the armature 8 of FIG. 4, prevents axial vibration, but requires too much height for a smaller-sized receiver design. A modified version of this fold shown in an armature 10 (illustrated in FIG. 5) utilized in a known receiver, reduces the height requirement, but requires additional width.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.